As is known, the variability in the lift of a valve drive of this kind is created by virtue of the fact that the different elevations of the cams are each transmitted selectively to the gas exchange valve by means of a cam follower, which is conventionally of rigid design. To allow the respective elevation to be activated in accordance with the operating point, the cam piece is arranged on a support shaft for conjoint rotation therewith but so as to be movable, and is moved backward and forward between the axial positions in accordance with the number of cams.
A valve drive of the type in question with a multi-valve system and camshaft bearings that are central with respect to the cylinders is disclosed by DE 101 48 179 A1, for example. With just two cams per cam group, however, the variability in the lift of this valve drive is restricted to two stages.
Valve drives with greater variability, having three cams per cam group, are proposed in DE 10 2007 010 148 A1, DE 10 2007 010 149 A1, DE 10 2007 010 150 A1 and DE 10 2007 027 979 A1. One significant common feature of these valve drives is that the camshaft bearing is a structural unit which is supported radially in the internal combustion engine, surrounds the bearing journal between the cam groups and, as a departure from the “traditional,” i.e. rigid, camshaft hearing assembly, is moved axially on the support shaft together with the cam piece. Despite the fact that, with three cams, the cam groups are relatively wide, together with the fact that the camshaft bearing assembly is central with respect to the cylinder and furthermore that the spacing between the gas exchange valves is small, a bearing design of this kind allows a sufficiently broad bearing journal support width that is independent of the axial positions of the cam piece.
The disadvantage is the associated outlay on production and assembly, especially because of the additionally introduced tolerances with respect to coaxial alignment at the movable camshaft bearing since the total tolerances in respect of the permissible radial runout of the camshaft have to be maintained within unchanged narrow limits.